Which of the following gemstones is uniquely made of a single chemical element throughout its crystal lattice structure?

Introduction / Context:
This question again links everyday gemstones with basic chemistry concepts about elements and compounds. Learners are asked to recognise which gemstone is built entirely from one element rather than from multiple elements in a compound. Such questions help bridge practical examples with the periodic table and improve the ability to classify substances correctly as elements, compounds, or mixtures.

Given Data / Assumptions:
- Four choices are presented: Platinum, Diamond, Emerald, and Ruby.
- The task is to select the gemstone that is a crystal of a single element rather than a chemical compound.
- We assume typical natural forms as used in jewellery and standard chemistry references.

Concept / Approach:
An element is a pure substance consisting of only one kind of atom. A gemstone may be an element, like diamond which is pure carbon, or a compound, like emerald and ruby, which are mineral compounds containing several different elements. Platinum is a metallic element used in jewellery, but it is usually considered as a metal rather than a transparent gemstone. Among the listed options, diamond is the classic and widely accepted example of a gemstone that is a pure element. The approach is to recall the composition of each material and then choose the one that is an elemental crystal.

Step-by-Step Solution:
Step 1: Consider platinum. Platinum is indeed a chemical element and a precious metal often used in rings and jewellery, but it is normally not classed as a gemstone, rather as a metal.
Step 2: Diamond is a crystalline allotrope of carbon, in which carbon atoms are covalently bonded in a three dimensional network. Every lattice point hosts a carbon atom, so the crystal is a pure element.
Step 3: Emerald is a variety of the mineral beryl, which is a compound containing beryllium, aluminium, silicon, and oxygen, with chromium or vanadium impurities giving the green colour.
Step 4: Ruby is a red variety of corundum, which is aluminium oxide with chromium impurities, so it clearly contains more than one element.
Step 5: Among the given options, diamond is both a gemstone and a pure element, so it is the correct choice.

Verification / Alternative check:
You can confirm this by consulting mineralogy or chemistry references. Diamond is always listed as an allotrope of carbon under elemental forms, while emerald and ruby appear as specific mineral compounds, not as elements. Platinum is listed as an element but appears under metals rather than gems. Therefore, if the question focuses on gemstones, diamond naturally stands out as the correct answer that fits both conditions.

Why Other Options Are Wrong:
- Platinum: It is a metallic element and a precious metal, but not a gemstone in the usual gemological sense used in such questions.
- Emerald: It is a mineral compound of beryllium aluminium silicate with impurities and therefore contains several different elements.

Why Other Options Are Wrong (continued):
- Ruby: It is aluminium oxide with chromium impurities and is not a single element crystal.

Common Pitfalls:
Some learners may pick platinum simply because it is an element, forgetting that the question asks specifically for a gemstone. Others might assume that all sparkling stones are directly elements. Another pitfall is not recalling the mineral compositions of emerald and ruby, which are classic examples of compounds. Carefully distinguishing between metals and gemstones and between elemental solids and mineral compounds will help avoid these mistakes.

Final Answer:
Diamond is the only gemstone listed that is made from a single chemical element, carbon, throughout its crystal lattice.